Abstract: This paper analyzes the mechanical mechanism of flash butt welding and designs a PLC control timing sequence based on the welding process. PLC control, through software programming on top of the hardware, improves control flexibility. PLC control ensures the stability of the control system and the welding quality.
Keywords: flash butt welding; PLC; mechanical mechanism; control system
introduction
Flash butt welding, as an advanced welding technology, boasts advantages such as no need for additional welding materials, high productivity, low cost, and ease of operation. With the continuous development of industrial technology, the cross-sections of welded parts are becoming increasingly larger, encountering several technical challenges, such as difficulty in heating, low productivity, and low product qualification rates. To address these issues in flash butt welding, many welding professionals have conducted a series of studies on the process, creating highly efficient and low-energy-consumption methods, such as pulsed flash butt welding and programmed voltage reduction flash butt welding. Controlling the flash butt welding process to maximize productivity while ensuring welding quality has always been our goal. Considering the factors affecting flash butt welding quality, this paper utilizes a PLC system to control the flash butt welding process, achieving the goal of welding quality control and thus improving the productivity of flash butt welding.
1. Mechanical Mechanism and Process Analysis
1.1 Mechanical device and operation process of flash butt welding
Figure 1 shows the mechanical device for flash butt welding. Its operation process is analyzed as follows:
1.1.1 Pre-adjustment
The preparatory work for flash butt welding includes adjusting the mechanical mechanism and selecting welding parameters. The main specifications for flash butt welding include: extension length, flash speed, flash current density, upsetting speed, upsetting pressure, and clamping force.
After debugging, the workpiece is mounted onto the worktable.
1.1.2 Clamping and Positioning
Press the start button, and the coils of solenoid valves PQ1, PQ2, and PQ3 are energized. Compressed gas flows through the three main components into the upper air chambers of clamping cylinders 1 and 2. The compressed gas pushes the piston rod downward to press the workpieces 1 and 2 until the clamping switch is closed.
Simultaneously, the gas flowing from the air pump enters the upper air chamber of the positioning cylinder 3 through the three main components, pushing the positioning rod upward to accurately position the workpiece. Once positioning is complete, the coil of the solenoid valve PQ3 in the flash butt welding mechanical device (Figure 1) is de-energized, and the positioning rod springs back.
1.1.3 Welding
Turn on the welding switch, keeping the coils of solenoid valves PQ1, PQ2, and PQ4 energized, while the coil of solenoid valve PQ5 is de-energized. Pressurized gas passes through the low-pressure components and enters the right chamber of the propulsion cylinder 4, pushing the piston rod and moving fixture to move workpiece 2 towards workpiece 1 until workpieces 1 and 2 contact and reach the preset position, at which point the propulsion switch closes. The moment workpieces 1 and 2 contact, heating begins. When the flash heating reaches the predetermined temperature, the coil of solenoid valve PQ5 is energized, and compressed gas passes through the high-pressure components to push the propulsion cylinder and moving fixture for rapid upsetting at high pressure. The welding current is then cut off and maintained for a period of time to allow the joint to cool and solidify. When the welding time is up, the welding switch is turned off, and the welding process ends.
1.1.4 Reset
When the coils of solenoid valves PQ4 and PQ5 are de-energized, the air path of the propulsion cylinder is reversed, and low-pressure gas enters the left air chamber of propulsion cylinder 4, pushing the cylinder and moving the worktable to the right. Propulsion cylinder 4 then resets. When the coils of solenoid valves PQ1 and PQ2 are de-energized, the air path is reversed, the clamping contacts spring back, and cylinders 1 and 2 reset. At this point, one flash butt welding process is complete, and all devices remain in their positions, ready to enter the next welding cycle.
1.2 Flash Butt Welding Timing Analysis
Because the actuator has many components and the actions of each component are sequential, a process timing diagram is first created to facilitate timing analysis. The flash butt welding process can be summarized as follows: pre-adjustment, positioning, clamping, feeding, welding, upsetting, holding, and resetting. Figure 2 shows the flash butt welding process timing diagram.
2. Implementation of PLC control process
2.1 Selection of PLC Model
A Programmable Logic Controller (PLC) is a new generation of industrial control device developed based on automatic control technology, microcomputer technology, and communication technology. It is currently widely used in various fields such as machinery, metallurgy, chemical industry, and welding. Based on factors such as the requirements of flash butt welding process and price, the Omron CPM1A series PLC was selected. This series of main units is available in four types according to the number of I/O points: 10, 20, 30, and 40. A 30-point PLC main unit was selected for the experiment, with a DC24 power supply and transistor outputs. This model has 18 input points (00000～00011, 00100～00105) and 12 output points (01000～01007, 01100～01003). It has a compact structure, strong functionality, and high cost-effectiveness, making it suitable for small-scale control.
2.2 PLC I/O Allocation
According to the requirements of the flash welding process, 17 input points (00003～00009, 00100～00107, and two high-speed counting input terminals 00000 and 00001) and 7 output points (01000～01006) of the PLC are occupied. The specific I/O allocation is shown in the table below.
2.3 Connection between PLC and peripheral circuits
Replacing time relays with programmable logic controllers (PLCs) essentially means replacing "hard" relays (actual components) with "soft" relays (programmable elements). To achieve this, the control requirements and operational processes in the original control system must first be analyzed. Based on a clear understanding of the various states and operational characteristics of the control process, the PLC's peripheral circuits are designed. Figure 3 shows a PLC peripheral circuit diagram designed according to the PLC I/O allocation table, which can accurately and conveniently control the flash butt welding process, achieving the goal of automatic control.
3. Conclusion
3.1 The mechanical device controls the flash butt welding process and upsetting process through two air paths: one for the high-pressure components and one for the low-pressure components. This ensures accurate workpiece advancement and meets the high-pressure requirements of the upsetting stage, providing convenient conditions for controlling the flash butt welding cycle. The entire process is easy to operate and highly mechanized.
3.2 Unlike traditional relay control, the PLC control system applied to the flash butt welding process is simple in circuitry, easy to use and maintain, and has high control precision. It realizes the mechanization and automation of the welding process while ensuring the flexibility and safety of the operation, and has broad application prospects in the welding industry.
References
[1] Zhao Xihua. Pressure Welding [M]. Beijing: Machinery Industry Press, 1999.
[2] Gong Shuzhen, Wang Dongqing, Xu Shixu. Programmable Controller Principles and Applications [M]. Beijing: Posts & Telecom Press, 2002.
